In machines using sensors, it is often necessary to trigger an output upon the sensor signal's crossing a certain level. A simple example is the reading of bar codes using an optical sensor. If the background of a particular bar code varies in reflectivity, or if the illumination varies during the scanning of the bar code, for example, a system with a single fixed trigger level or even a single programmable trigger level can be subject to false triggering or "jitter," causing a false reading of the bar code. Other sources of noise can also contribute to false triggering and resultant reading errors.